Peeing in the pool: gross, sure, but no real harm done? Think again. Researchers at Purdue and China Agricultural universities recently studied what happens when urine meets water containing chlorine and other chemicals—and they found that two potentially dangerous chemical byproducts are formed, Discover reports.

The uric acid in urine reacts with the chlorine to form cyanogen chloride and thrichloramine, which can affect the lungs, heart, central nervous system, and other organs if inhaled. io9 reports that the researchers also dosed water already containing those compounds with more uric acid, and found the cyanogen chloride level rose as a result.

Per the study, “Uric acid … accounted for 24% to 68% (depending on temperature, water pH and chlorine condition) of the byproduct cyanogen chloride in the pool water samples.”

Both chemicals have been linked to health problems experienced by swimmers, lifeguards, and pool staff, reports Quartz, which points to a 2012 survey that found 19% of adults cop to having peed in a public pool.

If you don’t count yourself among the group of offenders, you may be contributing to the problem anyway: Philly.com cites previous studies that suggest swimmers typically “drip out” as much as two shot glasses of urine when swimming.